Basically using a code injection plugin you can inject your own custom css and javascript to manipulate or change the UI in ways that no plugins or default configs allow for. I'm currently just using it to style things in ways which the current theme tokens don't allow for, but eventually I'd like to explore actually directly manipulating things with javascript. Though at that point I should probably just fork vscode itself or perhaps write a plugin for it. Though I believe plugins are limited to what the extension API allows for and I don't think code injection has that limitation. Not sure.